I think the point was to "bake" the 50% light into the backdrop using a shader that responds to lights. Then - load the saved render into a one-sided or as the Poser background, and set it up so that it is self-lit, i.e. just plug the rendered background into white Alternate_Diffuse or plug into white Ambient_Color and set Ambient_Value = 0, and turn off the lighting stuff (Diffuse_Value = 0, Specular_Value = 0).

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)